Overcurrent protective system for a direct-current load circuit



April 1948- D. E. MAXWELL 0, 08

OVER-CURRENT PROTECTIVE SYSTEM FOR A DIRECT-CURRENT LOAD CIRCUIT Filed May 5, 1943 Inventor Donald E. Maxwell,

His Attorney.

Patented Apr. 20, 1948 UNITED STATES PATENT OFFICE Donald E. Maxwell, Scotia, N. Y., assignor to Gen. eral Electric Company, a corporation of New York Application May 5, 1943, Serial No. 485,745

3 Claims. 1

I'he present invention relates to a relay circuit for producing a time delayed operation under sudden fluctuations. One use of such a relay circuit is in connection with an overload relay for a load circuit having a high inrush current oi short duration. Without; the time delayed operation, the overload relay might be operated by the inrush current under conditions when there was no overload over a period of time suificient to cause injury to apparatus in the load circuit.

The object of my invention is to provide an improved relay circuit for obtaining time delayed operation of a relay.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention, itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understocdby reference to the following description taken in connection with the accompanying drawing in which the single figure is a diagram of my relay circuit applied to an overload relay for a rectifier.

Referring to the drawing, there is shown a rectifier 6 having output terminals 2 connected to a load represented by a resistance 3 and a condenser i. The rectifier is fed through a transformer it having input terminals 6 connected to an AA power supply through a normally open switch or circuit breaker l. The switch has an operating coil 8 in series with a battery i through normally open contacts it controlled by a start button l l and normally closed contacts 112 of an overload relay 53. By momentarily pushing the start button ii, the coil is is energized to close the switch i and to close contacts it completing a holding circuit for the coil 8 through contacts it normally closed by a stop button it. Once the holding circuit is completed, the start button may be released and the switch 77 remains closed until either the contacts it: are opened by the stop button or the contacts B2 are opened by operation of the overload relay l3.

Upon closing the switch l, rectified current fiows to the load. If the condenser i is large, the inrush current (limited primarily by the resistance between the output terminals 2 and the condenser 4), though of short duration, will be much larger than the normal load current. A quick acting current overload relay connected directly in the load circuit would be operated by the inrush current, causing the switch 1 to open. By holding the start button closed, the switch I would bereclosed as soon as the contacts I! lit] Sit

a reclosed, and after one or more cycles the inrush current would be reduced by the charging of the condenser 4. The repeated operation (chatterlug) of the overload relay imposes an unnecessary load on the relay contacts.

in the present circuit, a time delay in the operation of the overload relay i3 is obtained by connecting the overload relay coil ii in parallel with a condenser iii, the condenser being connected in series with a resistance 58 across a variable resistance All in the load circuit. From the resistance it, a voltage corresponding to the load current is impressed on the time delay network consisting of the resistance is, condenser to, and the resistance and inductance of coil IT. The resistance and inductance of coil it should be relatively high, 1. e., coil ll has many turns of small diameter wire. This renders the relay oi the sensitive D.-C. type, having a very low value of pick-up current. chosen so that the network is non-oscillatory and has a time delay in the buildup of current through the coil ll sufficient to prevent operation of the overload relay by inrush currents which do not last long enough to endanger the rectifier. Under a gradually increasing load current, the charge on the condenser l8 and the current through the relay coil ll follow the load current the overload relay operates to open the con tact it (and cause opening of the switch 7!) as soon as the overload current reaches the value corresponding to the design of the relay and the setting of the variable resistance it (the smaller ta resistance 2c the larger the overload current required to operate the relay). Under a suddenly increasing load current, such as the initial inrush current or the momentary overload caused by sparking of a tube in the load circuit, the current through the overload relay coil it cannot instantaneously follow the load current and does not reach a value suficient to cause operation of the relay unless the overload current persists at least as long as the delay time determined by the con- StaZltS of the relay circuit. The relay has inverse time characteristics, i. e., the greater the overload current the faster the relay operates to open contacts l2. The same time delay is present between opening and reclosing of the contacts it. After opening of the contacts II, the contacts 52 are held open for a short time by the discharge of the condenser 58 through the relay coil I 7. If the start button I 0 is held closed and the load is short-circuited, the overload relay will successively open and close the contacts l2, causing the opening and closing of the switch I.

The circuit values are 3 The length or time the switch I is closed will be such that the average current will be insumcient to cause damage.

This relay circuit permits the use quick acting overload relays which are lighter and cheaper than time delay relays. The time delay and the overload setting are readily adjusted respectively by' changing the circuit constants and by adlusting the resistance 20.

What I claim as new and desire to secure by Letters Patent oi the United States is:

1. A rectifier control circuit, comprising a rectifier having input and output circuits, a circuit breaker in said input circuit, an operating coil for the circuit breaker in series with normally closed contacts oi an overload relay, a start switch for selectively closing the operating coil circuit, a coil for the overload relay, a condenser in shunt with the relay coil and both the coil and condenser being in series with the rectifler output circuit through a resistance, said relay coil. condenser, and resistance being proportioned to comprise a non-oscillatory circuit and so as to obtain a time delay in energizing the relay coil by sudden fluctuations in the rectifier output. I

2. In combination, an input circuit, a direct current load circuit coupled to said input circuit, a capacitor connected in series circuit relation with a resistor across a portion oi said load circuit, a relatively quick-acting overload relay having an energizing winding'connected directly in parallel circuit relation with said capacitor and in series with said resistor across said portion of said load circuit, said capacitor, resistor and winding forming a non-oscillatory timing circuit providing a predetermined delayed response to 4 sudden overload current variations in said load circuit. and means controlled by said relay for disabling said input circuit.

3. In combination, an alternating current input circuit, a direct current load circuit coupled to said input circuit and including a rectifier and a capacitive load element in series circuit relation, a relatively quick-acting overload relay operable to disable said input circuit and including a current responsive winding connected in series with a resistor across a portion of said load circuit, and a capacitor connected in parallel circuit relation with said current responsive winding and in series with said resistor across said portion of said load circuit, said winding, resistor and capacitor forming a non-oscillatory timing circuit eflective to delay the response of said relay to sudden overload variations in current in said load circuit and also to delay the release oi said relay after removal of said overload variations.

DONALD E. MAXWELL.

REFERENGES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 991,189 Barnum May '2, 1911 1,680,410 Crichton Aug. 14,- 1928 1,877,446 Curtis Sept. 13, 1932 1,901,628 Brainard Mar. 14, 1933 1,909,471 Kelly May 16, 1933 2,053,445 Rose Sept. 8, 1936 2,182,637 Marbury Dec. 5, 1939 2,210,669 Johnson Aug. 6, 1940 

